1. Field of the Invention
The present invention relates to the field of cold forming tubular materials and in particular to an apparatus and method for forming a complex-shaped tubular member from a tubular blank.
2. Description of the Prior Art
Appliance door handles and the like are currently constructed from multiple handle sections. For example, a typical oven door handle consists of three sections--a main tubular member and two curved end pieces which overlap the ends of the tubular portion and are connected thereto. The overlap of material between the end pieces and the tubular member results in a large amount of scrap material, especially when the number of appliance door handles produced each year is considered. Also, the additional step of connecting the end pieces to the main tubular member increases the time required to construct each door handle.
Therefore, in order to decrease the amount of material and labor consumed by the conventional production process discussed above, efforts have been made to develop a technique for forming one-piece appliance door handles. Since appliance door handles typically have curved ends, such a technique requires the bending and bulging of a relatively small-diameter tubular blank. Therefore, conventional development efforts have focused on two- and three-step processes for forming one-piece appliance handles. Unfortunately, as described below, these processes have disadvantages which limit their applicability to the production of appliance door handles and the like.
The general operations of bending, stretching, depressing and radially expanding a tubular blank, with or without a mandrel, are known. For the majority of metals, it is fairly easy to bend small diameter tubing into an arc having a large radius. But as the diameter of the tubing increases and the radius about which it to be bent decreases, the tube bending process requires some combination of compression at the inner bending radius of the tube and stretching at the outer radius. Although the outer bending surface of the tube may be stretched to the full extent of the material's rated elongation characteristics, one cannot satisfactorily bend a tube with a given diameter about a relatively small bending radius without encountering severe buckling at the inner bending surface or undesirable deformation at the outer bending radius. Some have been able to bend tubes with a certain diameter about relatively small bending radii by controllably dimpling or allowing controlled rippling of the inner tube surface, thereby creating less stretching of the outer tube surface.
Other examples of methods for bending a tube are disclosed in U.S. Pat. No. 4,704,886, which shows internally pressurizing a tube blank, gripping the opposite ends of the blank and applying longitudinal tension at the ends while applying a lateral force against the blank to bend the blank. U.S. Pat. No. 4,567,543, discloses depressing regions of the tube blank and then expanding the blank within a complementary shaped cavity formed by a pair of dies. U.S. Pat. No. 4,829,803 discloses forming a box-like frame member by internally-pressurizing a preformed tubular blank, closing a pair of die halves around the blank to partially deform the blank within mating die cavities, and then increasing the internal pressure to exceed the yield limit of the wall of the blank to expand the blank into conformity within the mating die cavities.
It is therefore desirable to provide an apparatus and method for forming a tubular blank into a tubular member having variations in the vertical and horizontal profiles, and in the cross-sectional configuration, thereof. Such an apparatus and method would result in substantial money savings for appliance door handles and the like.